Fibre optic current sensors typically are employed in high voltage electricity generating/distribution systems, and they make use of the Faraday effect to provide a measure of current flow through a conductor. Counter-propagating light signals are directed into opposite ends of a coil of the optical fibre that surrounds the current conductor and signal detecting/processing circuitry is provided to detect a phase shift between the polarisation modes of the counter-propagated light beams. This is achieved alternatively by using a polarising beam splitter and detecting directly for the phase shift or by inferring the phase shift from the interference of the counter-propagating light beams in a coupling device.
The current sensors rely upon the accurate measurement of the optical power levels of the light signals that have counter-propagated through the coil. However, the power levels are influenced not only by the electric current but, also, by changes in power level of the light source and by attenuation in the optical fibre leading to and from the coil. Also, electronic processing and measurement of the optical signals may give rise to errors due to changes in receiver sensitivity, amplifier gain and offset. Therefore, to stabilise absolutely the measurement of optical power levels would be an impossible task, even if consideration was to be given only to one or two influencing factors such as temperature induced variations and aging effects.